Optical Coherence Tomography in Cerebral Amyloidosis

Being a direct extension of the central nervous system and the only place in the human body where the vessels of the central circulation can be visualized directly, the eye provides a unique window to investigate the central circulatory system. Several studies have demonstrated that retinal blood vessels show structural and functional alterations in patients with dementia. These measurements are based on fundus pictures and are hence limited to the larger retinal vessels.

Until recently, intravenous injection of a contrast agent was necessary to visualize the retinal microvasculature in detail. While indispensable for the diagnosis of some ocular vascular diseases (arterial/venous occlusion, neovascularization,...) the invasiveness of fluorescein angiography (and the risk of an allergic reaction) limits its use as a screening tool to detect alterations in the microvascular network of the retina and choroid.

Optical coherence tomography (OCT) is a non-invasive diagnostic tool capable of generating cross-sectional coupes of the retina and choroid. Novel algorithms allow to render a 3-dimensional model of the ocular microcirculation based merely on the motion contrast of the circulating blood. Since OCT is fast, easy to perform and completely non-invasive, this technique lends itself for screening purposes.

Auto-fluorescence retinal imaging is an imaging modality that is commonly used in ophthalmological practice to assess retinal pigment epithelium and photoreceptor function by quantifying the relative amount of auto-fluorescent lipofuscin. The images can be acquired using the OCT device but require pupil dilation. Using visible light of 488nm, it is a safe imaging technique with no short or late term side-effects to the patient. A recent study has described abnormal auto-fluorescence patterns in patients with cerebral amyloid deposition.

Hyperspectral fundus photography is comparable to regular fundus photography but uses an image sensor that can acquire images at multiple different wavelengths (unlike the classical red-green-blue colour sensors that are used in conventional cameras). Apart from a different image sensor, the device is identical to a regular fundus camera. Recent studies have emerged that describe a unique hyperspectral signature of aggregated retinal amyloid deposits.